Maleic anhydride is staple article of commerce used in the production of polyesters. It is prepared, inter alia, through the catalytic oxidation of butane, available for example from petroleum refinery streams. See, e.g., U.S. Pat. No. 3,864,280. The catalyst of choice for this oxidation is one comprising vanadium, phosphorus, and oxygen, generally known as a vanadium/phosphorus oxide oxidation catalyst or phosphorus-vanadium mixed oxide catalyst.
It has been proposed that vanadyl (IV) pyrophosphate, (VO).sub.2 P.sub.2 O.sub.7, is the active phase in the selective oxidation of n-butane to maleic anhydride. The pyrolytic precursor to vanadyl (IV) pyrophosphate is believed to be vanadyl (IV) hydrogen phosphate hemihydrate, or VOHPO.sub.4.(H.sub.2 O).sub.0.5. This material has long been recognized as a layered material, the --POH groups forming interlayer hydrogen bonds and the water molecule being shared by two vanadyl octahedra which are face-linked.
A variety of such catalysts and methods for their production have been described.
U.S. Pat. No. 3,864,280 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst in which the vanadium has an average valence of from +3.9 to +4.6, the P:V ratio is from 0.9:1 to 1.8:1, and the B-phase content is at least 25%.
U.S. Pat. No. 4,043,943 describes forming a phosphorus-vanadium-oxygen precursor by calcining an intermediate obtained by reacting a vanadium compound with phosphorus compound in an oxygen-containing solvent containing less than 20% water.
U.S. Pat. No. 4,100,106 describes a process in which a water precipitated salt complex formed from tetravalent vanadium salt and orthophosphoric acid is calcined at a temperature of at least 300.degree. C.
U.S. Pat. No. 4,116,868 describes a catalyst prepared by calcining a phosphorus-vanadium-oxygen precursor formed in the presence of a small amount of surfactant.
U.S. Pat. No. 4,132,670 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst in which the vanadium has an average valence of from +4.0 to +4.5, the V:P ratio is about 1:1, and the intrinsic surface area is from 1 m.sup.2 /g to 10 m.sup.2 /g.
U.S. Pat. No. 4,288,372 describes a catalyst similar to that set forth in U.S. Pat. No. 3,864,280 but having a surface area greater than 10 m.sup.2 /g and containing lanthanum as a promoter.
U.S. Pat. No. 4,337,174 describes forming a phosphorus-vanadium-oxygen precursor which, prior to calcining, is heated at 130.degree. to 170.degree. C. and dried.
U.S. Pat. No. 4,418,003 describes the formation of a phosphorus-vanadium oxide catalyst by reacting a vanadium compound with phosphorus pentoxide in an acidic alcoholic medium.
U.S. Pat. No. 4,769,477 describes an attrition-resistant phosphorus-vanadium oxide catalyst incorporating silicon dioxide.
Various other catalysts are specified in, for example, U.S. Pat. Nos. 4,062,873, 4,333,853, and 4,562,268.